Clinicians have a critical and unmet need for readily available assays to identify those who are (or are not) responding to aspirin -- the most commonly used antiplatelet agent to prevent acute coronary syndrome (ACS). As a consequence, clinicians cannot tailor antiplatelet therapy. The rationale for this work is that platelet function in response to aspirin demonstrates variability, reproducibility, heritability, ad at high levels, association with ACS. However, platelet function tests are inaccessible to the vast majority of patients and study participants due to technical complexities. In contrast, whole blood RNA profiling overcomes these limitations and is currently widely available. In keeping with our long-term goal to personalize platelet inhibitors, this proposal's overall objective is to develop accessible, whole blood RNA biomarkers that can identify individuals at high risk for ACS on aspirin. Compelling preliminary data surrounding a unique set of coexpressed platelet transcripts discovered in whole blood RNA motivate this proposal. The central hypothesis is that aspirin exposure can identify aspirin-responsive platelet transcripts that, when assayed in whole blood RNA, can identify individuals at heightened risk for cardiovascular events due to suboptimal responses to aspirin. Under Aim 1 we will use RNA sequencing in healthy volunteers to identify platelet mRNA and microRNA transcripts associated with platelet function in response to aspirin and that are specifically responsive to aspirin exposure. Under Aim 2, we will characterize aspirin-responsive transcripts in patients at risk for ACS versus healthy volunteers in platelet and whole blood RNA to classify those who are (or are not) at risk for ACS on aspirin. Under Aim 3 we will assay aspirin-responsive transcripts in whole blood RNA from 3000 symptomatic patients at risk for ACS participating in a randomized clinical trial of diagnostic testing strategies (PROMISE NCT01174550) and fully characterize their prognostic and discriminative abilities compared to traditional risk factors. The expected outcome from this proposal is a translatable, whole blood RNA biomarker that identifies individuals at risk for ACS on aspirin and who are candidates for tailored antiplatelet therapy. The proposed studies are innovative in their goal of creating a proven and translatable technology platform for the response to aspirin that could be widely available -- a new and substantive departure from the status quo of assays that are currently inaccessible to the vast majority of patients at risk for ACS. The proposed research is significant because it is expected to enable any clinician or researcher to identify an individual with a suboptimal response to aspirin. Our results are expected to have an important positive impact, because analogous to the International Normalized Ratio for warfarin, developing such an assay is a critical step towards a novel treatment paradigm where use of platelet inhibitors is tailored to prevent ACS. As a consequence, an accessible, diagnostic, prognostic, and potentially actionable tool is expected to result.